fuse: Initialize total_len in fuse_retrieve()

fs/fuse/dev.c:1357: warning: ‘total_len’ may be used uninitialized in this
function

Initialize total_len to zero, else its value will be undefined.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>